1. Field of Invention
The present invention relates to apparatus and methods for making injection molded thermoplastic parts, particularly gas assisted injection molding of parts having a smooth outer skin and a hollow core.
2. Prior Art
Various injection molding techniques have heretofore been proposed to use less material and achieve weight and cost reduction while maintaining structural properties and providing a smooth outer surface or skin that does not require sanding or other finishing. Blowing agents, including gas, can be used to provide a porous, foamed or cellular core and in some cases a hollow core. When gas is used to form a hollow core, the gas can be injected into the plastic melt stream at the nozzle or directly into the mold, preferably in a controlled manner to achieve the desired core structure.
U.S. Pat. No. 4,474,717 to Hendry discloses several apparatus and methods wherein gas is injected into the mold by means of a gas injection probe. A small amount of plastic is first injected into the mold to encapsulate a gas injection probe and thereafter gas is injected through the probe while injection of the plastic continues to form the desired core structure. At the end of the molding operation gas pressure in the mold is relieved by exhausting the mold cavity through the probe which acts as a decompression valve. A similar gas injection - decompression valve in the mold is also disclosed in U.S. Pat. No. 4,740,150 to Sayer. In both the Hendry and Sayer patents the gas injection probe or nozzle is shown mounted in that half of the mold opposite the mold half through which the plastic is injected, as from a reciprocating screw injection molding machine. Although injecting and/or exhausting gas into the mold cavity in the manner taught by the Hendry and Sayer patents may well provide the desired core structure, modification of each mold is required to accommodate the gas injection nozzle or probe. This may be expensive and involves care in selecting the location of the gas injection nozzle, particularly in multiple cavity molds and in retrofitting existing molds.
Other gas injection techniques have also been proposed wherein the gas or a foaming agent is introduced into the melt stream prior to the mold cavity at the nozzle of the plastic injector as shown in U.S. Pat. No. 4,101,617 to Friedrich or into the cavity after plastic injection by a special manifold as shown U.S. Pat. No. 4,136,220 to Olabisi. Again, modification of the nozzle or manifold may also be expensive and require changing the nozzle or manifold for different applications depending on the part being molded by that machine.
Other gas injection locations have also been suggested. U.S. Pat. No. 4,498,860 (Gahan) discloses an inclined retractable piston mounted in a mold half that can be extended to close off a reverse taper sprue passageway and thereby cut off the sprue. A small pipe coaxial with the piston is disclosed for injecting gas into the plastic material to flow with the plastic through the mold space. Here again rather elaborate modification of the mold is required to accommodate the holder for the sprue cut-off piston. The inclined orientation of the gas injection tube would undoubtedly cause uneven distribution of the gas in the plastic entering the mold and otherwise detract from effective gas injection.
Whether gas is injected into the mold as in the Sayer and Hendry patents or into the melt stream before the mold as in the Friedrich patent, the gas injection should be compatible with different gas injection systems to precisely control injection of the gas. This may require further modification of the nozzle or the mold which in turn adds to expense, particularly where the gas injection system is installed as a retrofit for an existing mold to make a previously solid part into a hollow core part.
In retrofitting existing injection molding equipment and molds, as well as with new equipment, various techniques have been proposed to more precisely control the gas injection and achieve the desired core structure repeatably over long production runs. One approach is described in general terms in the aforementioned U.S. Pat. No. 4,740,150 to Sayer and in British Patent Specification No. 2,139,548 referred to therein, wherein a preselected or measured volume of pressurized gas is injected into the mold during each molding cycle.
A process using what may be generally termed as preset pressure has also been proposed in Baxi European application, Application No. 87304002.6, filed May 5, 1987, published Dec. 12, 1987, Publication No. 0250080A2, Bulletin 87/52. With this process, as contrasted to the preset volume technique, the quantity of gas that is introduced into the mold is not directly measured but only the pressure of the gas is controlled. A gas supply source is provided along with gas pressurization means for pressurizing the gas to a preset pressure which is at least as great as the pressure at which the molten plastic material is introduced into the mold. A storage chamber is provided for storing gas at the preset pressure so that the gas is immediately available for use when injection of the plastic material is initiated. Gas pressure maintains the plastic against the surfaces of the mold cavity as the plastic cools and until the plastic can sustain the form dictated by the mold to provide an essentially hollow part. As set forth in European Patent Publication 0,250,080, prior to injection of the plastic, a high pressure gas storage tank is fully charged at the pressure preset for that molding operation. Just after plastic injection is initiated, high pressure gas from the storage tank is injected into the plastic melt steam by a feed chamber in the nozzle. The high pressure tank is charged and recharged by a pump controlled by a pressure switch so that sufficient gas in the high pressure tank is always available at the preset pressure. Asahi Dow Ltd. Japanese Application No. 120318/1973, filed Oct. 25, 1973, published Mar. 27, 1982, Publication No. 14968/1982, shows a similar arrangement for injecting gas via a high pressure piston or ram and injection inlet at the nozzle.
Although the preset volume and preset pressure processes described in the prior art may well provide improved results as contrasted to gas injection that is not as precisely controlled, both processes have disadvantages that detract from precise and repeatable control of the gas injection. In the constant volume process it is difficult to maintain repeatability over many molding cycles due to variations inherent with constant volume cylinder and piston arrangements caused by wear and other variations with time and extended use. In the preset pressure method using a high pressure storage tank that must be replenished, the preset pressure can and will vary during an injection cycle as gas is released from the tank and replenished by the pump.
Accordingly it is desirable to provide improved methods and apparatus for injection molding of hollow parts which overcome the foregoing and other difficulties while providing better and more advantageous overall results.